Controlled, area-selective deposition of electrically conductive metal is important for a wide range of electrical applications, particularly in the manufacture of semiconductor integrated circuits and other microelectronic devices. In the manufacture of microelectronic devices, metal deposition is used to fabricate metal features such as contacts, interconnects, and interlayer wiring in multilayer circuits.
In recent years, the dimensions of metal features in semiconductor integrated circuits have been made progressively smaller, and even further reductions in the dimensions are contemplated for the future. In addition, metal features in microelectronic devices have been made increasingly complex. For example, in certain multilayer circuits, metal connections with high aspect ratios are required between the layers of the circuits. In general, the increasing complexity and ever smaller dimensions of metal features in microelectronic devices strain or exceed the capabilities of conventional methods to produce such features.
In addition to metal features of small dimensions, present day semiconductor integrated circuits frequently have doping profiles which change abruptly over short distances. To maintain such abrupt doping profiles during the processing required to fabricate the integrated circuit, the maximum temperature to which the integrated circuit can be exposed must be limited to limit thermal diffusion of the dopants. In addition, interdiffusion between the metal of a metal feature on a semiconductor integrated circuit and the semiconductor material in contact with the feature can be a problem if the integrated circuit is heated to an excessively high temperature during fabrication. For certain semiconductor integrated circuit technologies, it is necessary to limit the temperature to which the integrated circuits are exposed during processing to temperatures of less than 350.degree. C. or so. Even lower maximum temperature limits may be desirable for the future.
Gold has been used to fabricate metal features in semiconductor integrated circuits because of its high electrical conductivity, excellent chemical stability, and low tendency toward electromigration. Since gold has a propensity to diffuse into silicon, an intermediate layer of tungsten or similar refractory metal is often used in silicon integrated circuits as a diffusion barrier between the gold of a gold metal feature and a silicon substrate on which the gold feature is located.
Gold films and wires for microelectronic devices have been produced by a variety of methods. Conventional vacuum evaporation and sputtering of gold have been used to produce gold metal features in microelectronic devices. Vacuum evaporation and sputtering are ordinarily line-of-sight physical techniques and require masks to accomplish area-specific deposition. In general, line-of-sight deposition techniques do not provide good conformal coverage. Moreover, because of the value of gold, gold which deposits on the mask is usually recovered, which results in additional process steps and tends to limit the life of the mask.
Certain conventional chemically driven methods for depositing gold are capable of conformal coverage and area selectivity without the use of a mask. For example, in the chemically driven technique of wet electroless deposition of gold, area selectivity can be obtained in certain cases through reactions which are specific to different substrate materials. See generally Y. Okinaka in Electroless Plating, Mallory and Hajdu, eds. (American Electroplaters and Surface Finishers Society, Orlando, Fla., 1990) pages 401 through 420. However, conventional electroless deposition solutions for area-selective gold deposition are multicomponent solutions which ordinarily include solvents, reducing agents and other desired constituents-as well as undesired impurities which are difficult, if not impossible, to remove. Exposure of microelectronic devices to such multicomponent solutions in the course of depositing gold in the device can lead to contamination problems.
U.S. Pat. No. 4,714,627 to Puddephatt and Treurnicht disclosed a chemical vapor deposition (CVD) method for depositing gold onto a target surface using methyl(trimethylphosphine)gold(I), trimethyl(trimethylphosphine)gold(III), and certain other volatile organogold(I) and organogold(III) complexes. The method of the patent involved vaporizing the organogold complex under a vacuum and directing the vaporized complex into contact with a target surface heated to a temperature at or above the decomposition temperature for the complex. The organogold complex decomposed upon contacting the heated surface to deposit gold. According to column 7, lines 29 through 46 of the patent, when a glass tube containing a target disk was heated along with the disk to above the decomposition temperature of a particular isocyano-gold complex, gold metal was deposited both on the disk and on the walls of the tube. It was indicated that selective plating of the target disk could be achieved by selective heating of the target disk only.
An article by Thomas H. Baum in Journal of the Electrochemical Society, volume 134, pages 2616 through 2619 (October 1987) disclosed a laser-induced chemical vapor deposition technique for achieving area selective deposition of gold. A laser beam was focussed on a substrate to serve as a localized heat source. Dimethyl gold acetylacetonate absorbed on or colliding with the surface underwent a thermal decomposition to metallic gold with the liberation of volatile reaction products. According to the article, the surface temperature profile induced by laser heating of the substrate controlled the deposition. In general, laser-induced chemical vapor deposition is satisfactory for writing gold features no smaller than the width of the beam, but is not practical for complex scale circuits because of the time required to trace out the desired circuit with the laser beam. In addition, conventional laser-induced chemical vapor deposition ordinarily cannot produce gold structures with dimensions as small as desired or with cross-sectional aspect ratios as high as desired for present-day microelectronic devices. Conventional laser-induced chemical vapor deposition also typically leaves an undesirably high concentration of carbon or other impurities in the deposited gold.
U.S. Pat. No. 5,019,531 to Awaya and Arita disclosed a deposition procedure wherein gold was deposited by chemical vapor deposition from various organometallic complexes of gold. The organometallic complex together with a reducing gas such as hydrogen was directed onto a heated substrate. According to the patent, for substrates which had both surfaces of metal or metallic silicide and surfaces of oxide or nitride, gold deposited only on the surfaces of metal or metallic silicide. However, the requirement for the simultaneous presence of an organogold comlex and a gaseous reducing agent complicates the control of the deposition process. In some cases, moreover, a reducing gas can adversely affect existing structures on a substrate.
A publication by Colgate et al. in Journal of Vacuum Science and Technology, volume A8, pages 1411 through 1412 (May/June 1990) disclosed the area-selective chemical vapor deposition of gold on tungsten patterned on silicon using triethylphosphine gold chloride (CH.sub.3 CH.sub.2).sub.3 PAuCl. An n-doped silicon wafer bearing a polycrystalline tungsten pattern was degreased with isopropyl alcohol vapor and then exposed to a hydrogen plasma while heated to 500.degree. C. in a chemical-vapor-deposition reactor. After the 500.degree. C.-heating/hydrogen-plasma pretreatment, the pressure in the reactor was reduced to 1.times.10.sup.-6 torr and the wafer in the reactor was exposed to triethylphosphine gold chloride vapor with the temperature of the wafer maintained at 500.degree. C. According to the publication, the triethylphosphine gold chloride decomposed to deposit gold on the tungsten, whereas the decomposition was inhibited on the silicon. Gold films were reportedly grown on silicon surfaces that had been treated in aqueous hydrofluoric acid prior to deposition. It was stated that native oxide on silicon, which should be stable in the presence of a hydrogen plasma, seemed to be a factor in inhibiting deposition, while elimination of tungsten oxide aided the film growth on tungsten. Growth of gold films both on tungsten and on glass at temperatures below 200.degree. C. was reported in the publication, with the growth on glass being characterized as much slower than the growth on tungsten. In general, the decomposition of triethylphosphine gold chloride is expected to yield gaseous chlorine-containing decomposition products, which are expected to be corrosive-particularly at the 500.degree. C. temperature for which selective gold deposition on tungsten patterned on silicon was reported in the publication.